Plasma method for disposing of waste material, and apparatus therefor

ABSTRACT

A method and an apparatus based on a reactor vessel ( 201 ) that is brought to a predetermined vacuum degree, in particular said vacuum degree set between 2 and 6 absolute millibar, an to a high temperature. A waste material is continuously introduced in the vessel ( 202 ), in particular a solid waste material ( 5 ) is introduced in such a way that a vacuum loss from the vessel ( 201 ) is prevented. Owing to the high temperature, waste material ( 5 ) forms a plasma that undergoes a molecular rearrangement, which gives origin to a substantially liquid residue that is collected in a bottom portion ( 204 ) of reactor ( 201 ), and to a gas ( 11 ). The gas ( 11 ), which mainly comprises carbon monoxide, hydrogen, carbon dioxide and steam, i.e. a typical syngas composition, is continuously taken away from the vessel ( 201 ) through a duct ( 12 ), and is sent to subsequent treatments to obtain a gas fuel that can be burnt by devices such as a gas turbine, a motor or a boiler. The high temperature in reactor ( 201 ) is maintained due to a current in the form of electric discharges ( 20 ) that is established between two electrodes ( 21,22 ) below the liquid head ( 7 ). The substantially liquid residue is periodically discharged to maintain a liquid head height ( 7 ) not exceeding a reference height value H*, in such a way that air inlet into vacuum loss from the vessel ( 201 ) is prevented.

FIELD OF THE INVENTION

The present invention relates to a method for disposing of a waste material, which provides a transformation of the waste into a plasma. The invention also refers to an apparatus for carrying out the method.

The method and the apparatus are well suited for treating a solid, liquid or gas waste material, which may contain an organic and an inorganic part. In particular, the waste material may be a municipal waste, a hospital or surgery waste, a harmful or infected waste, a wastewater sludge, a metal workshop waste, a slaughterhouse waste, a farm waste, a ground tires, etc.

BACKGROUND OF THE INVENTION

The need is felt of waste disposal techniques, which overcome the well known drawbacks of traditional techniques, like landfill and incineration, and possibly allow recovering energy.

In particular, methods are known that provide heating a waste material up to a temperature at which the organic part of the waste material turns into a ionized gas, i.e. a plasma. In such conditions, the most complex organic molecules are broken down into lower molecular weight molecules, mainly carbon monoxide and hydrogen. These form a mixture that can be fed to a boiler, a gas turbine, or an endothermic motor to recover energy.

Waste materials may also contain a part that cannot be turned into a gas. This part generally comprises metals, in particular heavy metals, which at such a temperature are transformed into a lava, i.e. into a liquid that hardens by cooling, and gives origin to a glassy, hard, high-melting and not leachable material. The lava provides a matrix that is able to embed harmful compounds that may be present in the original waste material; solidified lava can be also used as a construction material.

WO2004/087840 relates to a two-stage process for disposing of a waste material, and to an apparatus to carry out the process, in which:

-   -   in the first stage, a waste material is heated up to a         temperature at which a plasma is obtained from the waste, a well         as a liquid residue that comprises a molten metal, partially         treated waste material and a slag. This occurs preferably in a         furnace into which the waste material is dropped, and where the         plasma is maintained by the heat that is produced by an electric         current that circulates between the residue and two immersed         electrodes;     -   after a rough solid separation, in the second stage, the gas         product is refined by further breaking residual complex         molecules; furthermore, carbonaceous particles, which may have         been formed in the first stage, are removed by contacting them         with a metered amount of an oxidizing agent like air and/or         steam.

Still in WO2004/087840, the steps are provided of quick cooling and/or purifying the gas; these steps are carried out by conventional methods and through a conventional equipment, in particular, one or more filters. A prefixed vacuum degree is created inside the furnace and throughout the gas circuit by means of a fan; the vacuum is merely intended to prevent the gas from leaking out of the furnace and the circuit through possible weak couplings. A vacuum higher than what is required to this purpose could not be used in the equipment according to the above mentioned application, as confirmed by the square cross section of the furnace, which is not adapted to work in deep vacuum conditions.

Furthermore, the furnace described in WO2004/087840 is not well suited for being steadily fed with solid waste material, without appreciable vacuum loss and/or appreciable air inlet. In fact, the feeding duct is just equipped with a “isolation valve”, i.e. an on-off valve, which cannot ensure a gas-tight seal. Similarly, it is quite difficult to feed such a furnace with hermetic, rigid capsules containing dangerous waste material, which would not be safe to open before feeding the furnace, as in the case of infected or harmful waste materials like hospital or surgery waste materials, and the like.

The continuous or periodic removal of the molten residue that is collected in the bottom part of the furnace is complicated by the high temperature that is needed to maintain the liquid state. Even if “holes for valves” are mentioned, the problem of the lava hardening inside the valves remains unsolved.

Briefly, a solution is still to be found to the problems of:

-   -   continuously feeding a loose or packed substantially solid waste         material into a transformation vessel; without substantial         vacuum loss and/or air inlet;     -   periodically removing the liquid mass, in particular, without         substantial loss of vacuum and/or air inlet.

SUMMARY OF THE INVENTION

It is therefore a feature of the present invention to provide a method and an apparatus for treating a waste material by a transformation into a plasma, in which energy costs are lower with respect to the prior art.

It is also a feature of the invention to provide such a method and such an apparatus, which allows energy recovering.

It is also a feature of the invention to provide such a method and such an apparatus, for obtaining useful products from the transformed waste material.

It is, furthermore, a feature of the invention to provide such a method and apparatus in which the waste material is continuously fed to a vessel where the transformation takes place, without substantial loss of vacuum and/or inlet of air along with the fed material.

It is a particular feature of the invention to provide such a method and apparatus that allow feeding a loose waste material or rigid capsules containing a waste material into the vessel where the transformation takes place.

It is, furthermore, an a feature of the invention to provide such a method and such an apparatus that allow drawing away a part of residual liquid from the vessel without stopping the treatment and without substantial loss of vacuum and/or inlet of air.

It is still, an a feature of the invention to provide such a method and such an apparatus that allow restoring portions of a consumable electrode, without stopping the treatment and without substantial loss of vacuum and/or inlet of air.

These and other objects are achieved through a method for transforming a waste material, the method comprising the steps of:

-   -   prearranging a vessel, the vessel having a bottom part, an inlet         duct that is associated to an inlet port, and an outlet port;     -   heating the vessel to a prefixed conversion temperature;     -   depressurizing the vessel to a prefixed vacuum degree, and         maintaining the prefixed vacuum degree by suction downstream of         the outlet port;     -   feeding the waste material into the vessel through the inlet         duct, the waste material being thus heated to the conversion         temperature, the conversion temperature being such that the         waste material turns into a ionized gas or a plasma, and into a         substantially liquid residue, in particular a lava, the liquid         residue forming a predetermined liquid head with respect to the         bottom part,     -   maintaining the conversion temperature by causing electric         discharges to occur in the vessel, in particular below the head         through the substantially liquid residue,     -   extracting the gas from the vessel through the outlet port,     -   extracting a part of the liquid residue through the outlet port,         such that the liquid head is maintained between prefixed minimum         and maximum levels,         wherein:     -   the steps of feeding the waste material and extracting the gas         are continuous,     -   the step of maintaining the prefixed vacuum degree is obtained         by the step of extracting the gas, and     -   the step of feeding the waste material into the vessel through         the inlet duct is carried out under vacuum conditions within the         vessel.

This way, less energy is required to maintain a plasma torch created by an electric arch in a vessel, provided that vacuum conditions are maintained in the vessel, in such a way that the current between the electrodes in intermittent, i.e. electric discharges take place between the electrodes.

Advantageously, the prefixed vacuum degree is comprised between one and ten absolute millibar, preferably between three and six absolute millibar. This way, a surprising efficiency of the electric discharge is obtained as compared to the costs for maintaining the vacuum in the vessel.

In particular, the step of feeding the waste material under vacuum conditions provides feeding a loose and substantially solid waste material, forming a gas-tight plug within the duct by compressing the waste material, the plug protruding into the conversion vessel through the inlet port.

Alternatively, the step of feeding the waste material under vacuum conditions provides feeding a waste material that is packaged into rigid capsules, the capsules forming a gas-tight engagement with the inlet duct while they are pushed towards the inlet port.

Alternatively, but not exclusively, the step of feeding the waste material under vacuum conditions provides feeding a liquid or a gas, or a combination thereof, under the head of liquid residues.

Preferably, a step of determining the level of the head is provided by ultrasonic analysis.

Advantageously, the step of extracting a part of the liquid residue is carried out through an outlet hole of the vessel through the bottom part, a step being provided of eliminating a plug that is formed by a hardened amount of the liquid residue at the outlet hole, in particular, the plug is eliminated by withdrawing it to a prefixed extent into the vessel such that it melts and opens the outlet hole, the plug restored when the liquid head decreases down to a prefixed minimum level.

Preferably the electric discharges take place in a discharge region below the head close to a plurality of conductive elements, in particular two electrodes, a prefixed voltage being established and maintained between a pair of the elements, wherein the electrical discharges are produced with an intensity and/or a frequency responsive to the voltage.

Advantageously, the voltage is adjusted responsive to an inlet throughput of the waste material into the vessel.

In particular, the conductive elements are consumable electrodes; in this case, the method provides detecting the length of the residual electrode and restoring a consumed portion of the electrode, in such a way that the discharge zone maintains its shape and its position while the electrodes are consumed. Alternatively, the method can provide continuously restoring the electrodes.

Preferably, the extraction step of the gas out of the vessel, which takes place through the outlet port, is followed by further gas treating steps.

In particular, a plasma purifying step can be provided, in which the gas flows between a couple of electrodes that are maintained at a predetermined voltage with respect to each other, such that an electric discharge takes place and creates or maintains a plasma torch, whereby harmful compounds are separated from the gas and fall down in a liquid form.

In particular, a quick cooling or quenching step of the outlet gas can be provided, such that the formation of harmful substances is prevented, such as dioxins and furans.

In particular, one or more filtering steps and/or a washing step of the gas can be provided to remove, in particular, hydrogen sulfide. The gas filtering steps can be carried out by such a means as a cyclone separator, and/or an electrostatic filter, or an activated carbon filter. The gas washing step can be carried out by means of solution of iron oxides or salts.

Advantageously, the plasma purifying, cooling, filtering and washing steps are carried out in high vacuum conditions.

The above-mentioned objects are also achieved through an apparatus for transforming a waste material, the apparatus comprising:

-   -   a vessel having a bottom part and an inlet duct that is         associated to an inlet port,     -   a heating means for heating the vessel to a prefixed conversion         temperature, and maintaining the vessel at the conversion         temperature,     -   a depressurizing means for depressurizing the vessel to a         prefixed vacuum degree, and maintaining the prefixed vacuum         degree,     -   a waste feeding means for feeding the waste material into the         vessel through the inlet duct, the waste material being thus         heated to the conversion temperature, the conversion temperature         being such that the waste material turns into a ionized gas or a         plasma, and into a substantially liquid residue, in particular a         lava, the liquid residue forming a liquid head with respect to         the bottom part. The apparatus comprises furthermore     -   a gas extracting means for extracting the gas from the vessel,     -   a liquid extracting means for extracting a part of the liquid         residue such that the head of the liquid residue is maintained         between prefixed minimum and maximum levels.

A main feature of the apparatus is that the depressurizing means is adapted to maintain in use the prefixed vacuum degree within the vessel during the gas extracting step, and that the waste feeding means is adapted to continually feed in use the waste material into the vessel and to create a gas-tight seal against air leakage together with the waste material.

The waste feeding means can be a means for feeding a loose and substantially solid waste material, the material forming a gas-tight plug within the duct by compressing the waste material, the plug protruding into the conversion vessel through the inlet port.

Alternatively, the means for feeding the waste material can provide a packed waste feeding means that is suitable for feeding rigid capsules containing a waste material, in particular a solid waste material, the capsules forming a gas-tight engagement with the inlet duct while being pushed towards the inlet port.

Alternatively, but not exclusively, the means for feeding the waste material can be a liquid/gas waste feeding means, or a means for feeding a combination of liquid and gas waste material, below the head of liquid residues.

In particular, the loose solid waste feeding means comprises a screw having a profile that is suitable for engaging the inlet duct in such a way that the loose solid material is compressed forming a gas-tight plug when it is fed into the vessel through the inlet duct, and the liquid/gas feeding means comprises a duct that protrudes inside the vessel and is arranged at a level below the head.

Preferably, the vessel is defined by walls that comprise:

-   -   an inner layer that is made of refractory bricks, which that         have respective engaging portions that are adapted to engage         with corresponding portions of adjacent refractory bricks;     -   an intermediate flexible layer made of a refractory material,         preferably made of aluminium oxide;     -   an outer coil, or a shell that is arranged about the         intermediate layer of the wall, an inner surface of the shell         and an external surface of the intermediate layer defining a         jacket, the coil or the jacket being adapted to receive a         cooling fluid for cooling the wall and/or for recovering heat         from the vessel.

Advantageously, the liquid extracting means comprises:

-   -   an outlet hole of the vessel;     -   a plug breaking means for breaking a plug that is formed by a         hardened amount of the liquid residue at the outlet hole.

The liquid extracting means can comprise a drilling means that is associated with a breaking detection means

Alternatively, but not exclusively, the liquid extracting means can comprise a screw that is arranged inside an extraction duct, the outlet duct being arranged through a wall of the vessel, the screw being kept in a first position within the outlet duct by a plug that is formed by a solidified amount of the residue, the screw adapted to assist the extraction by rotating towards the inside of the vessel.

Advantageously, the vessel comprises:

-   -   a plurality of conductive elements, in particular two         electrodes;     -   a voltage maintaining means for maintaining a prefixed voltage         between a pair of the elements of the plurality of elements, in         order to generate the electrical discharges in a region that in         use is submerged by the substantially liquid residue, the         electrical discharges having an intensity and a frequency, the         intensity and frequency responsive to the voltage.

Advantageously, the apparatus comprises:

-   -   an inlet throughput detecting means for detecting an inlet         throughput of the waste material in the vessel;     -   a voltage adjustment means for arranging the voltage responsive         to the throughput;         such that a heat input associated with such discharges can be         adjusted responsive to the throughput by adjusting the voltage.

The conductive elements can be consumable electrodes, each of the consumable electrodes formed by a plurality of substantially linear elements, each of the elements having ends, in particular screwed ends that are adapted to engage with another adjacent element, wherein an electrode restoring means is provided for restoring a consumed part of the electrode, the electrode restoring means comprising:

-   -   a positioning seat, for positioning an electrode element close         to an electrode end, the electrode end protruding out from the         vessel,     -   an electrode joining means for joining an end of the element to         the electrode end,     -   an actuator for pushing the electrode through the wall of the         vessel,     -   a gas-tight seal such that during the electrode progression air         leakages into the vessel are hindered.

The gas extracting means is preferably connected to a high temperature gas post-treating apparatus, as above described. This is preferably a tubular vessel, which is so-called eductor, which has a gas inlet port and a gas outlet port. A water or steam dosing means is provided at the inlet port of the eductor, as well as a couple of electrodes is provided between which a voltage is applied that is suitable for changing a gas that is flowing between the electrodes into a plasma. The outlet port of the eductor is connected to a quick cooling means, for example to a heat exchanger, which is suitable for quickly cooling down the gas, such that the formation of such harmful substances as dioxins and furans is prevented.

Preferably, a purification means is provided to remove hydrogen sulphide from the extracted gas, in particular a means for causing the gas to contact with a solution of ferrous oxides and/or salts.

At the end of the gas circuit, a vacuum device is provided for creating and maintaining suitable vacuum conditions inside the vessel and the circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be made clearer with the following description of an embodiment thereof, exemplifying but not limitative, with reference to the attached drawings wherein:

FIG. 1 is a flow-sheet that shows the steps of the method according to the invention;

FIG. 2 shows schematically a vessel where a transformation of a waste material takes place;

FIG. 3 is a cross sectional view of a screw conveyor for feeding the vessel of FIG. 2 with substantially loose or conventionally packed waste material;

FIG. 4 is a cross sectional view of a feeding means for waste material that is packed inside rigid capsules;

FIGS. 5′ to 5″″ show the liquid extracting means according to the invention, and the method for extracting a prefixed part of the substantially liquid residual material from the vessel.

FIG. 6 is a shows schematically the device for restoring the electrodes inside the vessel;

FIG. 7 shows schematically a vessel that is similar to the vessel of FIG. 2, as well as a device for post-treating and exploiting the fuel gas that is obtained from the vessel, including gas combustion and exhaust gas cleaning.

DESCRIPTION OF PREFERRED EXEMPLARY EMBODIMENTS

With reference to the flow diagram of FIG. 1, a method 100 is described for transforming a waste material, according to the invention. Method 100 comprises a step 110 of prearranging a vessel, a step 111 of heating the vessel up to a conversion temperature, which is normally set between 1600 and 2000° C., as well as a step 112 of depressurizing the vessel to an absolute pressure, which is preferably set between 1 and 10 millibar, and most preferably about 4 millibar. Depressurizing step 112 is followed by a step 120 of introducing the waste material into the vessel; step 120 is carried out in such a way that air is prevented from entering into the vessel together with the waste material, which would cause vacuum loss and combustion of a part of the waste material. The conversion temperature is high enough to cause and maintain a transformation step 130 of the waste material into two fractions, a ionic gas 11, i.e. a plasma, and a substantially liquid fraction 9 that is accumulated in a bottom part 204 of vessel 201, forming a liquid head height H (FIG. 2).

The waste material can be a substantially solid and loose material 5, which is continuously fed into vessel 201 in a compact form, for instance by means of a converging screw conveyor 40 (FIG. 3) to prevent air from leaking into vessel 201 together with waste material 5, which would cause vacuum loss. The method also provides feeding into vessel 201 a waste material that is packed in rigid capsules 51 a-d (FIG. 4); in this case, a gas-tight engagement is provided between capsules 51 a-d and a plug conveyor 300 to prevent air from entering into vessel 201. The method provides also feeding a liquid and/or gas waste material 6, below liquid head height H that accumulates in the bottom part 204 of vessel 201.

During transformation 130, heat is needed to maintain the conversion temperature. The heat is supplied by electric discharges 20 that take place in substantially liquid residue 9 collected in bottom part 204 of vessel 201.

Along with step 130, an extracting step 140 is provided for extracting gas 11 from vessel 201, followed by a plasma-purifying step 141, in which residual harmful compounds and impurities are separated from gas 11, which is still maintained in the state of plasma.

Step 141 is followed by a step 142 of quick cooling or quenching gas 11, in order to prevent the formation of dioxins, furans and other partially oxidized harmful substances, and by one or more final purifying steps which may comprise one or more filtering steps 143; a washing step 144 may also be provided to remove such compounds as hydrogen sulphide from gas 11. Finally, a step 145 may be provided of feeding purified gas 11 to a conversion device, for example a boiler, in order to recover heat from the process, or to a gas turbine or an endothermic motor, to obtain electric energy.

Referring now to liquid residue 9, a step 150 is provided of detecting liquid head height H (FIG. 2); the detected height value is sent to a human operator or to an automatic control unit, not shown, which carries out a step 151 of comparing detected head height value H and a prefixed reference head height value H*. Step 151 enables to take a decision whether a step 152 of opening the vessel must be actuated, in order to allow a subsequent step 153 of extracting a part of substantially liquid residue 9 from vessel 201, in order to maintain or restore reference height value H*, which is set between a prefixed minimum head height value and maximum head height value.

Step 152 is preferably carried out by breaking or removing a plug 26 of hardened liquid residue 9 through an outlet mouth 19 in the wall 28 of bottom part 204 of vessel 201 (FIGS. 2, 5′); at the end of step 153, a step 154 of blocking the outlet mouth or the hole 19 of vessel 201 is carried out by spontaneous hardening of an amount of substantially liquid residue 9 at outlet mouth 19, which forms a new plug 26 (FIG. 5″″).

Step 150 can be carried out, for example, by means of an ultrasonic level sensor 30 (FIG. 2).

Electric discharges 20, which allow heating and maintaining the conversion temperature, take place below the head of the substantially liquid residue, between two electrodes 67 and 68, among which a predetermined voltage V is maintained. The intensity and/or the frequency of electric discharges 20 depend/s upon voltage V. The method provides furthermore, a step of adjusting voltage V responsive to the feed rate of waste material 5 or 6, or responsive to a parameter that is representative of the feed rate, such as the speed of a screw 46 or of a plunger that causes the introduction of waste material 5 into vessel 201.

The electrodes, in particular graphite electrodes, are normally subjected to consumption during the process. For this reason, a step 160 of detecting the length of residual electrode is carried out along with step 130; for example, a length of the portion of electrode that is immersed in the liquid, or the portion present in vessel 201. The residual length value is sent to a human operator or to an automatic control unit, not shown, which carries out a step 161 of comparing residual length value E and a prefixed residual length reference value E*. Step 161 enables to take a decision whether a step 162 of adding an electrode element must be performed, in such a way that reference value E* is restored, along with a normal position of the electric discharge region. If two electrodes are present, one of them is normally consumed more quickly than the other, which is taken into account in step 162 still in order to maintain the shape and the position of the electric discharge region.

FIG. 2 shows vessel 201 of an apparatus that carries out the method 100. Vessel 201 is vertically arranged, and comprises a middle cylindrical part 202, and two round end portions 203 and 204. The round shape of the end portions, associated with a suitable wall thickness, allows creating high vacuum conditions inside vessel 201, as provided by the method 100, without any risk of implosion.

For the introduction of solid waste material 5, a gas-tight introduction means 10 is provided that prevents air or gas passage from a storage place, not shown, of waste material 5, into vessel 201. Gas-tight introduction means 10 is selected according to the nature of solid waste material 5, as shown in FIGS. 3 and 4, respectively referring to a loose waste material, and to a waste material that is packed in rigid capsules 51 a-e.

For the introduction a solid or liquid waste material 6, a duct 13 is provided, which has an outlet end preferably below a normal head of substantially liquid residue 9 that is contained in vessel 201. Along duct 13 a flow meter is provided, which produces a gas/liquid flow rate signal that is used by a control unit 23 to adjust voltage V between electrodes 66 and 67.

Due to the exposition to the conversion temperature and to the substantial absence of oxygen in vessel 201, waste materials 5 and 6 undergo a molecular rearrangement, by which a gas 11 is formed, as well as a substantially liquid residue 9, which is collected in the bottom part 204 of vessel 201. Substantially liquid residue 9 comprises a lava that derives mainly from an inorganic portion of waste materials 5 and 6, and that has a very high specific weight, normally about ten with respect to water specific weight. By hardening, residue 9 forms a glassy, hard, high-melting and not leachable material that is able to embed harmful compounds that may be present in the original waste material in particular, heavy metals.

Gas 11 evolves from waste material 5 while falling down inside vessel 201, and from substantially liquid residue 9. Gas 11 derives from an organic portion of waste materials 5 and 6, and contains mainly carbon monoxide, hydrogen, together with minor amounts of carbon dioxide and water. Gas 11 is therefore adapted to be used as a fuel in a conversion device, to recover energy from the process. On the other hand, gas 11 may contain solid particulate, comprising mainly inorganic materials like metals, as well as carbon that formed in the vessel due to the high temperature to which the organic portion of waste material 5 and 6 is exposed. Gas 11 is continuously extracted through a duct 12, and is sent to a sequence of purifying devices (FIG. 7) to obtain a gas, such as a syngas, that is suitable to be burnt in an energy conversion device like a boiler, a gas turbine, or an endothermic motor.

The conversion temperature in vessel 201 is ensured by the heat that is produced during electric discharge 20. For causing such discharge, a voltage V is applied between electrodes 21 and 22 by a voltage generating means 23. Voltage generating means 23 receives a feed rate signals 24 from a feed rate measuring device 26 and a flow rate signal 25 from a flow rate measuring device 27; signals 25 and 26 are respectively related to solid waste material 5 feed rate and to liquid/gas waste material 6 flow rate, and are used by generating means 23 to adjust voltage V, and therefore frequency and/or intensity of discharge 20. This way, the heat that is produced by discharge 20 is appropriate to treat the amount of waste material 5 and 6 that is currently introduced in vessel 201.

Walls 28 of vessel 201 comprise a first layer that is made of bricks (not shown) that have respective engaging portions that are adapted to engage with corresponding portions of a plurality of bricks. The bricks are made of a refractory material, for example aluminium oxide. Walls 28 comprise furthermore a refractory flexible second layer. Outside the second layer a coil or a jacket is provided, which allows a cooling fluid to lap the outer face of the second layer, in order to limit the temperature of the wall and possibly to recover heat.

An ultrasonic level sensor 30 that emits ultrasonic pulses 31 is provided to detect head height H; such sensor is connected to a control unit 32 with a display 33 where current value of head height H can be read.

FIG. 3 shows a screw conveyor 300 for introducing a loose substantially solid waste material 5 into vessel 201, for example a municipal waste or a ground tire material. Screw conveyor 300 comprises:

-   -   a hopper 38 with a vertical axis 39,     -   an introduction duct 40 that is arranged below hopper 38, in         which a recess 41 is made that extends between an end 42 that         communicates with hopper 38 through passage 43 and an end 44,         opposite to end 42, which communicates with vessel 201 through         an inlet port 56. Recess 41 is convergent towards end 44         according to a predetermined conical shape.     -   a screw 46 that is pivotally arranged within recess 41 and         passage 43, with a helical profile 49 whose outer diameter         decreases according to the conical shape of recess 41.

This way, waste material 5, periodically or continuously renewed in hopper 38 by a hopper feeding means, not shown, is continuously conveyed by screw 46 through recess 41 into vessel 201. While being conveyed, waste material 5 is shrunk and compacted along recess 41 and, at the same time, it is heated by contacting hot wall of duct 40, up to provide a compact and crumbly mass that prevents air from leaking into vessel 201 through hopper 38 and recess 41, even if end 44 is at a pressure less than atmospheric pressure. Once inlet port 56 has been achieved, the mass of waste material 5 falls down inside vessel 201 and disintegrates. Screw 46 is conventionally supported, for instance by a ball bearing 48, which is located in the wall behind duct 40.

FIG. 4 shows a plug conveyor 400 for introducing into vessel 201 capsules 51 a-d in which a waste material is sealed, in particular a harmful waste material like a hospital or surgery waste, and which would not be safe to open before feeding into vessel 201. Plug conveyor 400 comprises an introduction duct 52 that has a preferably cylindrical duct 53 with an inlet end 54 that receives capsules 51 a-e, and an outlet end 55, opposite to end 54, which communicates with an inlet port 56 of vessel 201. Capsules 51 a-e have preferably a central cylindrical portion, whose outer dimensions allow moving the capsules through duct 53 under the action of a force 57, and provide a gas-tight seal that prevents air inlet into vessel 52 together with waste material. In general, when a capsule 51 a-e enters into vessel 201 through outlet end 55, there is at least one capsule that forms the gas-tight seal at inlet end 54 of introduction duct 52; in the example of FIG. 4 there are two further following capsules 51 c-d that form the gas-tight seal.

FIGS. 5′ to 5″″ show an accumulation/extraction cycle of substantially liquid residue 9 in bottom part 204 of vessel 201. In particular, a screw conveyor 16 is arranged within an outlet duct 15, which is in turn arranged through a hole 19 of a wall 28 of vessel 201; screw conveyor 16 (FIG. 5′) is kept in a prefixed position within outlet duct 15 by a plug 26 formed by an amount of substantially liquid residue 9, which has been naturally hardened due to the exposition to room temperature outside vessel 201. Screw conveyor 16 is actuated by an electromechanical actuator, not shown, and is adapted (FIG. 5″) to break plug 26 by a rotation according to a prefixed direction 17; by rotating screw conveyor 16 in a direction 17′ opposite to direction 17 (FIG. 5′″) it is possible to assist extracting substantially liquid residue 9 from vessel 201. Once the head has fallen to a height value H₁, (FIG. 5″″), which is no longer enough to allow outflow of residue 9 through hole 19 and duct 15, plug 26 is naturally restored by the hardening of an amount of substantially liquid residue 9 that remains inside duct 15. Head H₁ is a head at which hydrostatic pressure of substantially liquid residue above duct 15 equals atmospheric pressure external to vessel 201, the internal absolute pressure of vessel 201 being negligible with respect to atmospheric pressure.

Electrodes 21 and 22 are preferably made of graphite, and are therefore consumed during transformation of waste material. In order to periodically restore the appropriate length of electrodes 20 and 21 without undesired air inlet and/or frequent treatment stops, each electrode is formed by modules 59/60, as shown in FIG. 6, which have screw threaded portions 68 for reciprocal engagement. and vessel 201 comprises an electrode restoring means 36, for restoring electrodes 21 and 22. Electrode restoring means 36 comprises a seat 63 for positioning a screw threaded module 60, in which module 60 can be rotated, and a means 61 for joining module 60 with modulus 59; electrode restoring means 36 comprises furthermore two couples of counter rotating wheels 58 for causing electrodes 21 or 22 to advance through wall 28 into vessel 201, and a gas-tight seal 64, which is engaged by electrode 21 or 22 during the movement towards inside vessel 201. In particular, wheels 58 can be operated automatically according to a program means residing in a control unit 35 (FIG. 2), which is suitable to set two different advancing speeds for electrodes 21 and 22, to maintain the relative position of zone 66 and 67 of electrodes 21 and 22 where electric discharges 20 take place.

With reference to FIG. 7, the gas extracting means 202 are connected to a post-treatment apparatus, which comprises a tubular vessel 210, called the eductor, which has an inlet port 211 and an outlet port 212. At the inlet port 212 a means 213 is provided for dosing water and/or steam in the gas flow, as well as two electrodes 214, among which a voltage V1 is applied suitable for changing the flow of gas 11 into a plasma torch, or to maintain the plasma state of gas 11. Outlet port 212 of eductor 210 is connected to a heat exchanger 220, which is suitable for quickly cooling or quenching gas 11, in order to prevent the formation of such harmful compounds as dioxins and furans. Heat exchanger 220 is followed by an absorber 230 where gas 11 comes into contact with an aqueous solution of ferrous oxide and/or ferrous salts, in order to remove, in particular, hydrogen sulphide H₂S.

After exchanger 220, conventional gas/solid separation means 240 are provided such as a cyclone separator, an electrostatic filter or an activated carbon filter. Once purified, gas 11 is sucked by a compressor or a vacuum pump 250 adapted to create a predetermined vacuum degree in vessel 201 and in the circuit engaged by gas 11. Gas 11 is then fed to a combustion chamber 260 of an energy converting device, for example a boiler to recover thermal energy or a gas turbine, or an endothermic motor to produce electric energy. Exhaust gas that flows out of the combustion chamber 260 is treated in a further eductor 270, preferably equipped with a gas diffuser 280 to diffuse the gas that is released to atmosphere.

The foregoing description of specific embodiments will so fully reveal the invention according to the conceptual point of view, so that others, by applying current knowledge, will be able to modify and/or adapt for various applications such embodiments without further research and without parting from the invention, and it is therefore to be understood that such adaptations and modifications will have to be considered as equivalent to the specific embodiments. The means and the materials to realise the different functions described herein could have a different nature without, for this reason, departing from the field of the invention. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. 

1. A method for transforming a waste material, said method comprising the steps of: prearranging a vessel, said vessel having a bottom part, an inlet duct that is associated to an inlet port, and an outlet port; heating said vessel to a prefixed conversion temperature; depressurizing said vessel to a prefixed vacuum degree, and maintaining said prefixed vacuum degree, by suction downstream of said outlet port; feeding said waste material into said vessel through said inlet duct, said waste material being thus heated to said conversion temperature, said conversion temperature being such that said waste material turns into a ionized gas, in particular a plasma, and into a substantially liquid residue, in particular a lava, said liquid residue forming a predetermined liquid head with respect to said bottom part, maintaining said conversion temperature by causing electric discharges to occur in said vessel, in particular below said head through said substantially liquid residue, extracting said gas from said vessel through said outlet port, extracting a part of said liquid residue through said outlet port, such that said liquid head is maintained between prefixed minimum and maximum levels, wherein: said steps of feeding said waste material and extracting said gas are continuous, said step of maintaining said prefixed vacuum degree is obtained by said step of extracting said gas, and said step of feeding said waste material into said vessel through said inlet duct is carried out under vacuum conditions within said vessel.
 2. A method according to claim 1, wherein said prefixed vacuum degree is comprised between one and ten absolute millibar, preferably between three and six absolute millibar.
 3. A method according to claim 1, wherein said step of feeding said waste material under vacuum conditions is selected from the group comprised of: feeding a loose and substantially solid waste material, forming a gas-tight plug within said duct by compressing said waste material, said plug protruding into said conversion vessel through said inlet port, feeding a waste material packaged into rigid capsules, said capsules forming a gas-tight engagement with said inlet duct while they are pushed towards said inlet port, feeding a liquid or a gas, or a combination thereof, under said head of liquid residues.
 4. A method according to claim 1, wherein a step of determining the level of said head is provided by ultrasonic analysis.
 5. A method according to claim 1, wherein said step of extracting a part of said liquid residue is carried out through an outlet hole of said vessel through said bottom part, a step being provided of eliminating a plug that is formed by a hardened amount of said liquid residue at said outlet hole, in particular, said plug is eliminated by withdrawing it to a prefixed extent into said vessel such that it melts and opens said outlet hole, said plug restored when said liquid head decreases down to a prefixed minimum level.
 6. A method according to claim 1, wherein said electric discharges take place in a discharge region below said head close to a plurality of conductive elements, in particular two electrodes, a prefixed voltage being established and maintained between a pair of said elements, wherein said discharges are produced with an intensity and/or a frequency responsive to said voltage.
 7. A method according to claim 6, wherein said voltage is adjusted responsive to an inlet throughput of said waste material into said vessel.
 8. An apparatus for transforming a waste material, said apparatus comprising: a vessel having a bottom part and an inlet duct that is associated to an inlet port, a heating means for heating said vessel to a prefixed conversion temperature, and maintaining said vessel at said conversion temperature; a depressurizing means for depressurizing said vessel to a prefixed vacuum degree, and maintaining said prefixed vacuum degree; a waste feeding means for feeding said waste material into said vessel through said inlet duct, said waste material being thus heated to said conversion temperature, said conversion temperature being such that said waste material turns into a ionized gas or a plasma, and into a substantially liquid residue, in particular a lava, said liquid residue forming a liquid head with respect to said bottom part, a gas extracting means for extracting said gas from said vessel a liquid extracting means for extracting a part of said liquid residue such that said head of said liquid residue is maintained between prefixed minimum and maximum levels, characterized in that said depressurizing means is adapted to maintain in use said prefixed vacuum degree within said vessel during said gas extracting, and that said waste feeding means is adapted to continually feed in use said waste material into said vessel and to create a gas-tight seal against air leakage together with said waste material.
 9. An apparatus according to claim 8, wherein said waste feeding means is selected from the group comprised of a loose solid waste feeding means for feeding a loose and substantially solid waste material, said material forming a gas-tight plug within said duct by compressing said waste material, said plug protruding into said conversion vessel through said inlet port, a packed waste feeding means for feeding rigid capsules containing a waste material, in particular a solid waste material, said capsules forming a gas-tight engagement with said inlet duct while being pushed towards said inlet port, a liquid/gas waste feeding means for feeding a liquid or a gas, or a combination thereof, below said head of liquid residues.
 10. An apparatus according to claim 9, wherein said loose solid waste feeding means comprises a screw having a profile that is suitable for engaging said inlet duct in such a way that said loose solid material is compressed forming said gas-tight plug when it is fed into said vessel through said inlet duct. said liquid/gas waste feeding means comprises a duct that protrudes inside said vessel and is arranged at a level below said head.
 11. An apparatus according to claim 8, wherein said liquid extracting means comprises: an outlet hole of said vessel, a plug breaking means for breaking a plug that is formed by a hardened amount of said liquid residue at said outlet hole, said means selected from the group comprised of a drilling means that is associated with breaking detection means, a screw that is arranged inside an extraction duct, said outlet duct being arranged through a wall of said vessel, said screw being kept in a first position within said outlet duct by a plug that is formed by a solidified amount of said residue, said screw adapted to assist said extraction by rotating towards the inside of the vessel.
 12. An apparatus according to claim 8, wherein said vessel comprises: a plurality of conductive elements, in particular two electrodes, a voltage maintaining means for maintaining a prefixed voltage between a pair of said elements of said plurality of elements, in order to generate said discharges in a region that in use is submerged by said substantially liquid residue, said discharges having an intensity and a frequency, said intensity and frequency responsive to said voltage.
 13. An apparatus according to claim 12, comprising: an inlet throughput detecting means for detecting an inlet throughput of said waste material in said vessel a voltage adjustment means for arranging said voltage responsive to said throughput such that a heat input associated with such discharges can be adjusted responsive to said throughput by adjusting said voltage.
 14. An apparatus according to claim 12, wherein said conductive elements are consumable electrodes, each of said consumable electrodes formed by a plurality of substantially linear elements, each of said elements having ends, in particular screwed ends that are adapted to engage with another adjacent element, wherein an electrode restoring means is provided for restoring a consumed part of said electrode, said electrode restoring means comprising: a positioning seat, for positioning an electrode element close to an electrode end, said extremity protruding out of said vessel an electrode joining means for joining an end of said element to said end of said electrode, an actuator for pushing said electrode through said wall of said vessel, a gas-tight seal such that during said progression air leakages into said vessel are hindered.
 15. An apparatus according to claim 8, wherein a purification means is provided to remove hydrogen sulphide from said extracted gas, in particular a means for contacting said gas with a solution of ferrous oxides and/or salts. 